The Fascial Continuity–Memory–Reflex Coupling and the Fascial Integration Layer Surrounding the Carotid Sheath with Investing Fascia Involvement: A Hub Hypothesis

1. Introduction: From “Parts-Based” Thinking to a Holistic Network View

1.1. The “Parts-Based” Thinking of Traditional Medicine

Traditional medical education breaks the human body down into independent systems, such as the skeletal, muscular, nervous, circulatory, and lymphatic systems, and teaches, studies, and treats them separately. This decomposition approach facilitates teaching and clinical practice, but it also creates a fundamental blind spot: how these systems connect and influence one another is often overlooked.

1.2. The Core Proposition of the Holistic Network View

From a holistic perspective, the human body can be viewed as a continuous, adaptive, self-organizing mechanical–sensory–immune network. Bones provide rigid anchor points, muscles provide active contractile force, and fascia provides continuity, mechanical integration, sensory input, a memory substrate, and a system-coupling interface. Together, they form an inseparable tension-based whole.

1.3. The Rise of Fascial Research

Fascia is not a set of unrelated wrapping membranes; rather, it is a continuous connective tissue network that forms during embryonic development and extends throughout the body. A systematic review has confirmed that there is moderate evidence of intermuscular force transmission along myofascial chains, and that tension can be transmitted between adjacent structures [1]. However, the same systematic review also notes that no demonstrable transition zone was found for the Superficial Front Line in seven studies, that evidence for the Spiral Line and Lateral Line is only moderate, and that quantitative evidence for force transmission in the human body remains limited. Recent studies have identified CD34+ membranous cells in the subcutaneous fascia that highly express mechanosensors such as integrins, cadherins, and PIEZO, and that interweave with matrix fibers to form an intrafascial tension mechanosensory apparatus [2]. In addition, studies at the fascial tissue level in a mouse model of arthritis suggest that fibroblasts functionally express PIEZO1 and YAP1, and that selective ablation of these cells attenuates the therapeutic effects of mechanical stimulation [3]. This evidence is derived from a mouse model of arthritis, not from human fascial tissue.

1.4. Conceptual Definition: The Levels of the Term “Memory”

This hypothesis distinguishes four types of memory: procedural motor memory, which is stored in the central nervous system and mainly involves the basal ganglia, cerebellum, and motor cortex; conditioned reflex memory, which is encoded by the central nervous system; fascial mechanical memory, which refers to the ability of fascial tissue to retain the history of past mechanical stress through collagen remodeling, cell–ECM feedback dynamics, and mechanical epigenetic modifications; and neuroplasticity-related memory, which refers to the remodeling of innervation reflected by an increased number of nociceptors in pathological fascia. In this hypothesis, “fascial memory” specifically refers to the mechanical–structural memory of peripheral fascial tissue (defined here as the ability of fascial tissue to retain the history of past mechanical stress through structural changes) and does not replace or equate to central procedural memory or conditioned reflex memory.

1.5. Anatomical Controversy and Functional Definition of the Investing Fascia

Traditional anatomy describes the investing fascia (investing layer of the deep cervical fascia) as a continuous fibrous layer encircling the neck. However, Nash et al., using E12 plastinated sections and confocal microscopy for three-dimensional reconstruction of the anterior cervical triangle, showed that the so-called investing fascia does not exist in the anterior cervical triangle and that the deep potential spaces of the neck are directly continuous with the subcutaneous tissue [24]. Zhang and Lee’s three-dimensional histological analysis of the posterior cervical region showed that no fibrous connective tissue aggregation was observed between the sternocleidomastoid and trapezius muscles, and the investing fascia was described as incomplete [25]. It should be noted that the investing fascia is present on the surface of the trapezius and forms its muscle sheath; the discontinuity described by Zhang and Lee specifically refers to the fascial space between the sternocleidomastoid and trapezius muscles. Diwan et al. confirmed through dissection of 12 sides from 6 cadavers that the human investing fascia has a dense bilaminar structure, with the supraclavicular nerve running between its layers; ultrasound-guided injection between the layers with only 2 mL of dye effectively stained the nerve structures [26]. A recent developmental histology study further provided histological evidence of interlayer boundaries at the level of the carotid sheath: Zhao et al. performed histological analysis of the rabbit carotid sheath across four developmental stages and confirmed that a three-layered fascial architecture consistently exists around the common carotid artery, internal jugular vein, and vagus nerve, namely an inner fascial layer, a middle zone, and an outer fascial layer; the arterial inner fascial layer appeared earliest and thickened most markedly, whereas the outer fascial layer showed potential fusion, suggesting that intrinsic pulsatile stress and extrinsic mechanical interactions may jointly participate in the formation of this interlayer structure [27]. This evidence comes from a rabbit model and cannot be directly equated with the human fascia surrounding the carotid sheath, but it provides a quantifiable histological reference for the functional expression “fascial integration layer surrounding the carotid sheath.” Therefore, this hypothesis adopts the functional expression “fascial integration layer surrounding the carotid sheath” to refer to the convergence and coupling of fascial sheets from different regions, including the trapezius fascia, around the carotid sheath, rather than a single, anatomically completely continuous fascial layer.

1.6. The Integrative Framework and Core Proposition of This Hypothesis

This hypothesis integrates 47 lines of research evidence and organizes them into three thematic domains. At the anatomical level: fascial continuity [1], fascial anatomy surrounding the carotid sheath [24,25,26,27], carotid sheath histology [28,29,30,31], and fascial innervation [32,33,34,35]. At the molecular mechanism level: mechanical sensitivity of fascial resident cells [2], functional expression of PIEZO1/YAP1 in fascial fibroblasts (mouse model of arthritis) [3], expression of PIEZO1 and PIEZO2 in the carotid sheath region [4,6,22,36,37,38,39], YAP mechanotransduction pathways [7,15,16,17,18,19,20,21,40,41] (including animal-derived in vitro evidence [17,19]), and mechanical epigenetic modifications [23]. At the clinical and mechanical level: mechanical parameters of the deep cervical fascia [8,9], central memory and conditioned reflexes, peripheral fascial mechanical memory [42,43], clinical evidence for FHR [10,11,12,13,14,44,45], a systematic review of the thoracolumbar fascia [46], and mechanical transmission to the carotid sinus [5]. The core proposition is: central encoding is the primary component of memory and reflexes, peripheral fascia is the mechanical memory substrate and reflex modulator, and the fascial integration layer surrounding the carotid sheath may be a candidate strategic hub coupling the systemic fascial network with vital organs. The first-priority validation step is P1, namely directly confirming the expression of PIEZO1, PIEZO2, and YAP in resident cells of the human fascia surrounding the carotid sheath. If P1 is not confirmed, the fascial resident cell component would lack direct molecular support, although other cellular sources of PIEZO1/2 might still partially contribute.

2. The First Principle of the Holistic View: Fascial Continuity

2.1. Embryological Origin: Differentiation of a Single Network

The mesoderm forms loose mesenchymal cells that constitute a continuous network. Some of these cells differentiate into structures such as bones, muscles, blood vessels, and lymphatic vessels, while the mesenchymal cells that remain in place differentiate into connective tissues such as fascia, tendons, ligaments, and joint capsules. Differentiation does not sever this network. Bones are encased in periosteum, muscles in epimysium, perimysium, and endomysium, nerves in epineurium, perineurium, and endoneurium, and blood vessels in adventitia. These enveloping layers are continuous with one another through loose connective tissue. Embryologically, the fascia of the entire body is a single continuous membrane.

2.2. Longitudinal Continuity: From the Cranial Vertex to the Sole of the Foot

The longitudinal continuity of fascia can be traced from the cranial vertex to the sole of the foot. The galea aponeurotica at the cranial vertex continues downward as fascial tissue derived from the superficial layer of the deep cervical fascia; the deep cervical fascia connects downward with the thoracolumbar fascia, the thoracolumbar fascia continues downward as the gluteal fascia, the gluteal fascia connects with the fascia lata of the thigh, the fascia lata continues downward as the deep fascia of the leg, and the deep fascia of the leg ultimately connects with the plantar fascia. A systematic review of 9 moderate- to high-quality studies confirmed that moderate evidence of intermuscular force transmission exists in all three transition zones of the Superficial Back Line, and that tension can be transmitted between adjacent structures [1]. The same systematic review also noted that no demonstrable transition zone was found for the Superficial Front Line in seven studies, that evidence for the Spiral Line and Lateral Line is only moderate, and that quantitative evidence for force transmission in the human body remains limited.

2.3. Transverse and Cross-Connections: Spiral Mechanical Chains

The linea alba is a crossing weave of abdominal fascia at the midline. The serratus anterior fascia and the contralateral external oblique fascia form a spiral connection through the thoracolumbar fascia and the linea alba. Krause et al. showed that moderate evidence of force transmission exists in at least one transition zone of the Back Functional Line [1]. This suggests that the mechanical state of one limb may influence contralateral or distal structures through the fascial network.

2.4. Key Nodes

The thoracolumbar fascia is the “crossroads” of the systemic fascia, connecting the upper limbs, trunk, lower limbs, and pelvis. Its deformability is dynamically regulated by autonomic nervous system activity, with a delay of several days [42]. Schleip et al.’s systematic review further suggests that quantitative ultrasound biomarkers of the thoracolumbar fascia are associated with nonspecific low back pain, but that stiffness findings are heterogeneous across elastography modalities [46]. The diaphragmatic fascia connects the thoracic and abdominal cavities and is central to respiratory mechanics and trunk stability. The pelvic floor fascia is the “base” of the systemic mechanical chain.

The fascial integration layer surrounding the carotid sheath is one of the most important candidate fascial structures in the neck. Traditional anatomy describes the carotid sheath as a dense fascial conduit formed by fibers contributed by the investing fascia, pretracheal fascia, and prevertebral fascia. However, three-dimensional histological studies have challenged this description [24,25]. Diwan et al. further confirmed that the human investing fascia has a dense bilaminar structure, with the supraclavicular nerve running between its layers, and that only 2 mL of fluid injected between the layers can spread effectively [26]. Zhao et al.’s developmental histology study further confirmed the presence of a three-layered fascial architecture around the carotid sheath [27].

A study of 43 participants used a pressure algometer and ultrasound to measure the mechanical pressure required to reach the deep cervical fascia in humans; the results showed 6.06 ± 0.186 N/cm² in the cervical region and 5.85 N/cm² in the thoracolumbar region (median 7.287 N/cm²), suggesting a consistent mechanical response across anatomical regions [9]. The same study also found that the pressure pain threshold in the cervical region (18.88 ± 0.917 N/cm²) was significantly lower than that in the thoracolumbar region (46.46 ± 2.408 N/cm²), suggesting that the cervical fascial region has higher pain sensitivity to mechanical stimulation than the thoracolumbar region [9]. In addition, mechanical sensitivity and subcutaneous fat thickness did not affect the mechanical pressure required to reach the deep fascia [9]. Zandi et al. studied 30 participants (including patients with chronic neck pain and low back pain) and confirmed that the intraclass correlation coefficient (ICC ≥ 0.90) for cervical fascial stiffness was excellent, and they provided standard error of measurement (SEM), minimal detectable change (MDC), and Bland-Altman analyses [8]. In the forward head posture, posterior cervical fascial stiffness increases, but there is no side-to-side or sex difference [8]. Schleip et al.’s systematic review further suggests that quantitative ultrasound biomarkers of the thoracolumbar fascia are associated with low back pain, but emphasizes that these imaging differences cannot be directly equated with pathological remodeling or fibrosis and that their biological mechanisms remain hypothetical [46].

3. Bone–Muscle–Fascia: A Tension-Based Whole

3.1. Bones Are More Than a Scaffold

Bones provide rigid support and levers for muscle attachment, but they are not passive scaffolds. Osteocytes can sense mechanical stress and remodel trabecular bone structure according to the direction of loading. The periosteum itself is part of the fascial network. Bone is the mechanical anchor and rigid support of the fascia.

3.2. Muscles Are More Than Contraction

Muscles are responsible for active contraction, but they are not merely an “engine.” Muscle spindles and Golgi tendon organs continuously send information about length, tension, and velocity to the central nervous system. Muscle force is not transmitted to bone only through tendons; it also spreads to distant sites through myofascial chains. A systematic review has confirmed that intermuscular force transmission does exist and that tension can be transmitted between adjacent structures [1]. Muscle is an active contraction engine, but its force is integrated and distributed through the fascial network.

3.3. Fascia as an Integration Platform

Bones are segmented and muscles are compartmentalized, but fascia is continuous. Fascia is responsible for the integration and transmission of force and determines where force is transmitted, how much is transmitted, and how far it travels. Fascia is a body-wide sensory network, and the density of its sensory nerve endings is higher than that in muscle. Systematic reviews have confirmed that fascia is richly innervated, primarily by proprioceptors and nociceptors [32]. Fascia wraps nerves, blood vessels, lymphatic vessels, and organs and forms bridges for mechanical and signal transmission among these structures. Fascia is a candidate substrate for memory and adaptation. Its collagen remodeling and YAP-associated epigenetic changes may allow it to “remember” past mechanical environments, which is termed fascial mechanical memory. This concept remains hypothetical and requires direct molecular validation. Multiple mechanotransduction pathways can drive fibroblast activation, as detailed in Section 6.3.

In one sentence: bones are the foundation, muscles are the engine, and fascia is the network. Without the network, the foundation and the engine cannot work in concert effectively.

4. The Central Level: Procedural Motor Memory and Conditioned Reflexes as Global Regulation

The “memory” discussed in this section specifically refers to central procedural motor memory and conditioned reflex memory, both of which are stored in the central nervous system and differ from peripheral fascial mechanical memory.

4.1. Procedural Motor Memory

The core of “muscle memory” is actually procedural motor memory. When learning to ride a bike, swim, or touch type, the brain does not memorize the contraction sequence of each muscle; rather, it establishes an efficient motor program in the basal ganglia, cerebellum, and motor cortex. Repeated practice continuously optimizes the neural circuits among these regions, ultimately forming an automatic program that requires almost no conscious involvement. The key point is that this program is stored in the central nervous system, not in the muscles or the fascia.

4.2. Classical Conditioning

Classical conditioning was discovered by Pavlov. After a neutral stimulus is repeatedly paired with an unconditioned stimulus, presenting the neutral stimulus alone can elicit a conditioned response. In the context of fascia and pain, classical conditioning can explain many clinical phenomena. For example, after a patient experiences a neck sprain, a certain angle of head rotation is repeatedly paired with pain. Later, even after the tissue has healed, merely rotating the head to that angle can trigger pain anticipation and defensive muscle contraction.

4.3. Operant Conditioning

Operant conditioning was discovered by Skinner. In chronic pain, operant conditioning can explain the formation of postural habits and movement avoidance. When patients find that a certain posture can reduce pain, they repeatedly adopt that posture, eventually forming a compensatory postural pattern.

4.4. Regulation of Fascial State by Central Conditioned Reflexes

The central nervous system not only encodes motor programs and conditioned reflexes but also regulates the state of peripheral fascia through the autonomic nervous system and the endocrine system. Sympathetic activation can alter vascular tone and the contractile state of smooth muscle-like cells within fascia, whereas parasympathetic activation can promote tissue repair and anti-inflammatory responses. Chronic psychological stress or conditioned pain responses can continuously alter the mechanical and immune environment of fascia through autonomic pathways, promoting fibrotic remodeling. A 30-day continuous monitoring study showed that sympathetic nervous system activity significantly mediated the effects of daily stressful events on thoracolumbar fascia deformability, with a 2-day delayed effect followed by a parasympathetic-mediated compensatory overshoot [42]. This suggests that central conditioned reflexes are not merely a “psychological” phenomenon; they may alter the structure and function of fascia through autonomic and peripheral mechanisms. This evidence comes from a single-case report and represents human time-series evidence, but it remains preliminary. The regulation of fascial state by the central–autonomic–endocrine axis may be mediated by the renin-angiotensin system (RAS), and Ang II can directly activate YAP in fascial fibroblasts, driving fibrotic gene expression [16]. This evidence is derived from thoracolumbar fascia fibroblasts, representing extrapolation from a non-target tissue. The holistic view requires seeing both central encoding and peripheral effects simultaneously, rather than separating the two.

5. The Peripheral Level: Fascial Memory and Fascial Reflexes as Global Regulation

The “memory” discussed in this section specifically refers to peripheral fascial mechanical memory and neuroplasticity-related memory, both of which are located in peripheral tissues and differ from central procedural memory and conditioned reflex memory.

5.1. Hypothetical Mechanisms of Fascial Memory

Fascial memory does not mean that fascia stores episodic memories like the brain; rather, it refers to the ability of fascial tissue to retain the history of past mechanical stress through structural changes. In response to sustained mechanical tension, collagen fibers rearrange and are deposited along lines of tension, and this structural change itself can be regarded as a kind of “tension memory.” Sustained mechanical stress can alter the gene expression profile of fibroblasts, predisposing them to synthesize more collagen and degrade less matrix. This change in cell phenotype can persist for some time after the stress is removed, forming a memory at the level of the extracellular matrix. Hong et al. demonstrated that the long-term effects of static mechanical stretch on fibroblast activation in three-dimensional tissue depend on the feedback relationships among ECM viscoelasticity, signaling dynamics, and cell mechanics, yielding a predictable but non-monotonic stretch–activation relationship. Cell–ECM feedback determines how cells retain memory of mechanical stretch [43]. This evidence is derived from an in vitro three-dimensional tissue model.

Sustained mechanical stress can activate intracellular YAP/TAZ signaling pathways through mechanotransduction, thereby inducing epigenetic modifications. The key molecular node in this chain, YAP, has been directly confirmed in human deep fascia [15]. Multiple mechanotransduction pathways can drive fibroblast activation, as detailed in Section 6.3. At the human cell level, Liao et al. demonstrated in keloid fibroblasts that Piezo1 transmits mechanical signals to YAP through calcium flux [22]. Shim et al. demonstrated in human skin fibroblasts that mechanical stretch induces changes in chromatin accessibility and downregulation of H3K9me3 [23]. However, these two lines of evidence are derived from keloid fibroblasts and skin fibroblasts, respectively, not from fascial fibroblasts.

Systematic reviews have confirmed that innervation of pathological fascia increases and that the number of nociceptors increases [32]. Direct immunohistochemical analysis of the trapezius fascia by Tereshenko et al. further confirmed that the population of nociceptive axons increased by 71% in patients with occipital neuralgia, and that CGRP was significantly upregulated in the neural components of the fascia [31]. This evidence is derived from human trapezius fascia and constitutes direct human histological evidence.

5.2. Strict Definition of Fascial Reflexes

The “reflex coupling” in the title of this hypothesis does not mean that two parallel reflex systems exist. The central neural reflex is the sole integration center of the classical reflex arc, whereas fascial reflexes are the modulation and amplification of neural reflexes by fascia as a peripheral mechanical–chemical transduction network. The two form a “central encoding–peripheral modulation” coupling circuit, rather than two parallel independent reflex systems. Fascia cannot independently complete a classical reflex arc because it lacks central integration capacity. However, fascia can serve as a peripheral mechanical–chemical transduction network and participate in the following local circuits.

After sensory nerve endings are mechanically stimulated, impulses travel antidromically along axonal branches and release neuropeptides such as CGRP and substance P. Systematic reviews have confirmed that fascia is richly innervated, including nociceptors that express CGRP and substance P [32]. Recent studies have identified a class of CD34+ membranous cells in the subcutaneous fascia that highly express mechanosensors such as integrins, cadherins, and PIEZO and that are arranged in a lamellar morphology alternating with collagen bundles [2]. After mechanical stimulation activates these cells, it may trigger contraction and collagen remodeling, and the resulting change in local tension further activates mechanoreceptors. The long-term effects of this local circuit depend on the dynamic parameters of cell–ECM feedback [43]. Tu et al. confirmed at the fascial tissue level in a mouse model of arthritis that fibroblasts functionally express PIEZO1 and YAP1, and that selective ablation of fascial fibroblasts significantly attenuates the therapeutic effects of mechanical stimulation [3], further supporting the existence of an intrinsic cellular mechanical feedback circuit.

Sympathetic nerve endings are present within fascia and participate in the regulation of local vascular tone. A systematic review has confirmed the presence of tyrosine hydroxylase-positive sympathetic nerve fibers within fascia [32]. Mense’s semiquantitative analysis of thoracolumbar fascia innervation showed that approximately 40% of all innervation consists of postganglionic sympathetic nerve fibers [33]. The reverse pathway also exists: Brandl et al.’s time-series analysis showed that higher sympathetic activity was immediately associated with lower thoracolumbar fascia deformability, and that the reduction in fascial deformability caused by stressful events was mediated by the sympathetic nervous system, appearing with a 2-day delay [42].

Yahia et al. used anti-neurofilament protein and S-100 protein antisera to identify three structures in human thoracolumbar fascia surgical specimens: free nerve endings, Ruffini corpuscles, and Vater–Pacini corpuscles [34]. Tereshenko et al. further confirmed the presence of Pacinian-like and Ruffini-like mechanoreceptors in human trapezius fascia [31]. Woo et al. demonstrated that PIEZO2 is expressed in the sensory nerve endings of muscle spindles and Golgi tendon organs; knockout of Piezo2 in proprioceptive neurons caused severe motor incoordination and abnormal limb position in mice, and stretch-induced proprioceptor firing was significantly reduced in muscle nerve recordings, indicating that PIEZO2 is the principal mechanotransduction channel for mammalian proprioception [40]. This evidence is derived from mouse muscle spindles and Golgi tendon organs. Tesarz et al.’s quantitative assessment of rat thoracolumbar fascia distinguished three layers—outer, middle, and inner—and found that SP-positive free nerve endings were present only in the subcutaneous tissue and outer layer, whereas none were detected in the middle or inner layers; preliminary assessment of human thoracolumbar fascia showed a similar pattern [35]. This evidence is derived from rat thoracolumbar fascia.

Persistent abnormal mechanical signals generated by fascia can alter the firing patterns of afferent nerves, thereby modulating the threshold and intensity of central reflexes. Systematic reviews have confirmed an increased number of nociceptors in pathological fascia [32], and the findings of Tereshenko et al. in the trapezius fascia of patients with occipital neuralgia [31] further support this mechanism. It must be emphasized that fascia is not an independent reflex arc; without nerve endings and the central nervous system, fascia cannot complete classical conditioning or operant conditioning. What fascia can do is serve as a peripheral mechanical–chemical transduction network, participate in axon reflexes, modulate the threshold of neural reflexes, maintain mechanical memory, and convert repeated neural–immune–mechanical signals into structural changes. Neural reflexes and fascial reflexes are not two parallel systems. Instead, they constitute a coupling circuit of central encoding–peripheral modulation–mechanical memory.

6. Molecular Anchors: The Roles of PIEZO1/2 and YAP in Mechanotransduction

6.1. PIEZO1/2: Mechanosensitive Cation Channels

The 2021 Nobel Prize in Physiology or Medicine was awarded to David Julius and Ardem Patapoutian for their discoveries of TRPV1 and PIEZO1/2, respectively. PIEZO1 and PIEZO2 are mechanosensitive cation channels that open directly in response to changes in membrane tension and mediate Ca²⁺ and Na⁺ influx. In fascial tissue, PIEZO1 and PIEZO2 are presumed to be expressed on fibroblasts, resident membranous cells, and free nerve endings. Recent studies have detected high expression of PIEZO family genes in CD34+ membranous cells in the subcutaneous fascia; these cells are arranged in lamellar morphology alternating with collagen bundles, forming an intrafascial tension mechanosensory apparatus [2]. However, that study used subcutaneous fascia from the abdominal midline of rats, not the human fascia surrounding the carotid sheath, and thus cannot be directly extrapolated. Tereshenko et al. used Piezo2 markers in immunofluorescence analysis of human trapezius fascia and confirmed the presence of mechanosensitive structures within the fascia, with a morphology resembling Pacinian and Ruffini corpuscles [31]. However, the same study also noted that Piezo2 expression did not show a significant signal in the assay, suggesting that mechanoreceptors may exist in the form of Ruffini-like or Pacinian-like corpuscles rather than directly expressing PIEZO2.

Emig et al. confirmed in human atrial fibroblasts that Piezo1 is functionally expressed in human fibroblasts; altering Piezo1 expression levels can affect cytoskeletal organization and cell mechanical properties, and Piezo1 plays a key role in fibroblast adaptation to changes in matrix stiffness [36]. Zheng et al. confirmed in human lung fibroblasts that mechanical stretch increases Piezo channel expression, and that Piezo1 mediates mechanical stretch–induced ECM production and fibroblast-to-myofibroblast transition through ERK phosphorylation and Ca²⁺ influx; Piezo1 inhibitor GsMTx4 or PZ1 siRNA knockdown can block this effect [37]. Götschi et al. confirmed in humans that the PIEZO1 gain-of-function variant E756del is associated with significantly elevated tendon stiffness; carriers show approximately 46% and 45% increases in patellar tendon stiffness and Young’s modulus, respectively, indicating that PIEZO1 functional status can causally regulate the mechanical properties of dense connective tissue at the human level [38]. Tu et al. confirmed functional expression of PIEZO1/YAP1 at the fascial tissue level in a mouse model of arthritis [3]. Liao et al. confirmed in human keloid fibroblasts that Piezo1 transmits mechanical signals to YAP through calcium flux, increasing YAP nuclear translocation; knockdown of Piezo1 or YAP reduces CCN2 and CCN1 expression as well as fibrosis-related cellular behaviors [22].

6.2. Histological Evidence for PIEZO1/2 in the Human Carotid Sheath Region

In the sensory nerve structures of the human carotid sinus wall, immunohistochemistry and immunofluorescence combined with laser confocal microscopy have confirmed the expression of PIEZO1 and PIEZO2 proteins [4]. In the human superior cervical ganglion, approximately 71% of neurons are PIEZO1-positive and 86% are PIEZO2-positive [4]. The study concluded that PIEZO1 and PIEZO2 may participate in mechanosensation and/or mechanotransduction in the human carotid sinus [4]. This finding advances the existence of PIEZO channels in the human carotid sheath region from a hypothetical inference to the level of histological evidence. It should be noted that the study examined neural structures and non-neural cells in the carotid sinus wall, not fascial resident cells. Therefore, the expression of PIEZO1 and PIEZO2 in the fascial resident cells surrounding the human carotid sheath still requires direct validation, namely P1.

Konishi et al. performed Piezo1 immunohistochemical analysis on carotid plaques from 17 patients who underwent carotid endarterectomy and found that Piezo1 expression in myofibroblasts was significantly higher in symptomatic patients than in asymptomatic patients (1.78 ± 0.23 vs. 1.34 ± 0.17, P < 0.001), and that it was associated with plaque rupture, thin-cap fibroatheroma, and microcalcification [6]. This suggests that PIEZO1 is indeed expressed in fibrotic cells in the human carotid region, and that its expression level is significantly correlated with pathological status. Zhang et al. used tail suspension to simulate microgravity in rat and mouse models and found that the carotid artery developed aging-like changes, including increased stiffness, increased thickness, fibrosis, and elevated senescence markers; specific knockout of Piezo1 in vascular smooth muscle cells significantly attenuated these changes [39]. Mechanistically, stretch-induced Piezo1 activation elevates miRNA-582-5p, which enhances the synthetic cell phenotype and collagen deposition through the PTEN/PI3K/Akt signaling pathway [39]. This evidence is derived from animal models, and the cells examined were carotid vascular smooth muscle cells.

6.3. YAP: A Key Molecule in Mechanotransduction and Fibrotic Remodeling of Deep Fascia

YAP is a downstream effector of the Hippo signaling pathway and a key intracellular mechanosensitive transcriptional coactivator. Sustained mechanical stress can activate YAP, thereby promoting fibroblast activation, extracellular matrix deposition, and tissue stiffening. Pirri et al. were the first to confirm in human thoracolumbar fascia samples that YAP is expressed in deep fascia and that focused extracorporeal shock wave–induced mechanical stimulation can activate YAP [15]. Caroccia et al. demonstrated in human fibroblasts derived from the thoracolumbar fascia that angiotensin II activates YAP in fascial fibroblasts through its AT1 receptor—short-term exposure leads to YAP dephosphorylation and nuclear translocation, whereas sustained treatment significantly upregulates fibrosis-related genes such as COL1A1, COL3A1, and HABP2, and these effects can be blocked by AT1R antagonists or YAP inhibitors [16]. Tu et al. demonstrated in the L929 mouse fibroblast cell line that mechanical stretch activates the YAP pathway by upregulating Piezo1 expression, thereby promoting fibroblast activation, manifested as cytoskeletal reorganization, increased proliferation, and enhanced migration [17]. Huang et al. revealed another independent fibrotic pathway downstream of PIEZO1 in an epidural fibrosis model: PIEZO1 drives fibroblast activation and collagen synthesis through the ET-1/HIF-1α/TGF-β1/Smad3 axis, forming a positive feedback loop between collagen accumulation and fibroblast activation, and blocking PIEZO1 activation alleviates fibrotic progression in vivo [18]. Li et al. demonstrated in cultured rat bladder fibroblasts that hydrostatic pressure activates PIEZO1, which in turn activates YAP1, upregulates glutaminase 1 expression, enhances glutamine metabolism, and ultimately drives fibroblast activation and fibrotic gene expression [19]. Xu et al., using conditional gene knockout in a pulmonary fibrosis model, demonstrated that the PIEZO1–YAP/TAZ axis is functionally necessary for in vivo fibrosis: Piezo1 deletion led to disruption of actin organization and blocked Yap/Taz nuclear translocation, shifting myofibroblasts from a proliferative state to an apoptotic state; Yap/Taz knockout completely reproduced the protective phenotype of Piezo1 knockout [20]; however, Piezo1 deletion only partially attenuated YAP/TAZ activation, suggesting the existence of YAP-independent pathways.

Cui et al. performed YAP immunohistochemical analysis in human carotid plaque tissue and found that YAP was mainly distributed in plaque calcification areas, near neovascularization, and in the media; YAP protein expression was significantly higher in symptomatic patients than in asymptomatic patients (mean IOD: 3647.17 ± 229.45 vs. 3153.03 ± 178.69, P < 0.001); carotid bifurcation angle showed a positive linear correlation with YAP distribution in the symptomatic group (P < 0.001, R² = 0.68), and the internal carotid artery angle also showed a positive linear correlation (P < 0.001, R² = 0.74) [7]. Liao et al. confirmed the functional existence of the Piezo1–Ca²⁺–YAP pathway in human keloid fibroblasts [22]. Tu et al. confirmed functional expression of PIEZO1/YAP1 at the fascial tissue level in a mouse model of arthritis [3]. Wen et al. systematically reviewed the mechanisms and therapeutic prospects of the Piezo1/YAP axis, noting that this axis converts mechanical signals into pro-fibrotic transcriptional programs through Ca²⁺ influx, cytoskeletal remodeling, and YAP/TAZ nuclear translocation [21]. This suggests that YAP may be a convergence node for mechanical signals and biochemical signals (the RAS). PIEZO1, in turn, can drive deep fascial fibrotic remodeling in parallel through multiple YAP-dependent and YAP-independent pathways. PIEZO2 has been confirmed to be the principal mechanotransduction channel for proprioceptors [40], playing a central role in the functional implementation of mechanoreceptors within fascia.

6.4. Mechanical Epigenetic Modifications: From Human Fibroblasts to Fascial Memory

The hypothesis in Section 5.1 proposes that sustained mechanical stress can activate YAP/TAZ signaling pathways through mechanotransduction, thereby inducing epigenetic modifications that constitute a candidate molecular basis for fascial memory. Shim et al. provided direct experimental evidence for a key link in this chain in human skin fibroblasts: brief cyclic mechanical stretch can rapidly induce chromatin decondensation and nuclear softening, marked by decreased H3K9me3 levels; stretch activates globular actin assembly into perinuclear actin through Ca²⁺ released from the nucleus/endoplasmic reticulum, and perinuclear actin remodeling coordinates with H3K9me3 downregulation through emerin translocation at the nuclear envelope; genome-wide ATAC-seq analysis showed that mechanical stretch increases chromatin accessibility at gene loci related to mechanotransduction and DNA damage repair [23]. This study did not directly examine the role of YAP/TAZ or validate the findings in fascial fibroblasts, but it advanced “mechanical epigenetic memory” from a purely conceptual inference to a stage with an experimental anchor in human cells; this evidence is derived from skin fibroblasts, not fascial fibroblasts.

6.5. TRPV1: An Amplifier of Mechanical–Chemical Signals

TRPV1 has traditionally been defined as a sensor of temperature and chemical stimuli, but increasing evidence indicates that TRPV1 also participates in mechanical pain sensitization. Systematic reviews have confirmed the presence of nociceptors expressing CGRP and substance P within fascia [32], and the release of these neuropeptides is closely associated with TRPV1 activation. PIEZO1/2-mediated Ca²⁺ influx may sensitize TRPV1 through intracellular signaling pathways, lowering its activation threshold. Activation of TRPV1 releases substance P and CGRP, causing local vasodilation and neurogenic inflammation. CGRP has been shown to enhance the mechanical sensitivity of PIEZO1, potentially forming a positive feedback loop of “mechanical activation → TRPV1 sensitization → CGRP release → PIEZO1 sensitization.”

6.6. The Key Question of Cellular Localization

This hypothesis must answer a core question: are these channels and molecules expressed on resident cells of the fascia surrounding the carotid sheath, or do they exist only on nerve endings passing through the fascia? Existing evidence suggests that both possibilities exist. CD34+ membranous cells in the subcutaneous fascia highly express mechanosensors such as PIEZO [2]; Tu et al. confirmed functional expression of PIEZO1/YAP1 at the fascial tissue level in a mouse model of arthritis [3]; PIEZO1 and PIEZO2 proteins have been detected in the sensory nerve structures of the carotid sinus wall [4]; YAP is expressed and mechanically activated in human deep fascia [15]; and multiple PIEZO1–YAP pathways are activated in fibroblasts [16,17,18,19,20]. The above evidence suggests that both fascial resident cells and nerve endings may participate in mechanotransduction. However, these lines of evidence are derived from rat subcutaneous fascia, a mouse model of arthritis, sensory nerve structures of the carotid sinus wall, human thoracolumbar fascia, the L929 cell line, an epidural fibrosis model, rat bladder fibroblasts, and a pulmonary fibrosis model, respectively, not from resident cells of the human fascia surrounding the carotid sheath. Whether the two coexist remains to be distinguished by double-labeling immunohistochemistry. P1 remains the most critical open step in the entire hypothesis. A systematic overview of the evidence sources and gaps is provided in Section 10.4.

7. The Hub Level: The Fascial Integration Layer Surrounding the Carotid Sheath as a Global Coupling Node

7.1. Anatomical Definition of the Fascial Integration Layer Surrounding the Carotid Sheath

Traditional anatomy describes the carotid sheath as a dense fascial conduit formed by the fusion of fibers contributed by the investing fascia, pretracheal fascia, and prevertebral fascia. However, the literature provides complex, varied, and inconsistent descriptions of the fascial composition of the carotid sheath, its attachment to the skull base, and its extension into the mediastinum; the connection between the superficial layer of the cervical fascia and the carotid sheath is relatively loose, and blunt dissection can separate the two. Nash et al., using E12 plastinated sections and confocal microscopy for three-dimensional reconstruction of the anterior cervical triangle, showed that the so-called investing fascia does not exist in the anterior cervical triangle and that the deep potential spaces of the neck are directly continuous with the subcutaneous tissue [24]. Zhang and Lee’s three-dimensional histological analysis of the posterior cervical region showed that no fibrous connective tissue aggregation was observed between the sternocleidomastoid and trapezius muscles, and the investing fascia was described as incomplete [25]. It should be noted that the investing fascia is present on the surface of the trapezius and forms its muscle sheath; the discontinuity described by Zhang and Lee specifically refers to the fascial space between the sternocleidomastoid and trapezius muscles. Therefore, the “fascial integration layer surrounding the carotid sheath with investing fascia involvement” refers to the convergence and coupling of fascial sheets from different regions, including the trapezius fascia, around the carotid sheath, rather than a single, anatomically completely continuous fascial layer. Diwan et al. confirmed through dissection of 12 sides from 6 cadavers that the human investing fascia has a dense bilaminar structure, with the supraclavicular nerve running between its layers; ultrasound-guided injection of only 2 mL of dye between the layers can effectively stain the nerve structures [26]. Zhao et al.’s developmental histology study further confirmed a three-layered fascial architecture at the level of the carotid sheath, namely an inner fascial layer, a middle zone, and an outer fascial layer, with the arterial inner fascial layer showing the most marked thickening and the outer fascial layer showing potential fusion [27]. Based on these findings, this hypothesis adopts the functional expression “fascial integration layer surrounding the carotid sheath” to refer to the actual collection of fibrous connective tissue around the carotid sheath that originates from different fascial sources. This expression acknowledges that the continuity of the cervical fascia may not be achieved in the form of a single investing layer, but rather through functional mechanical coupling between different fascial sheets or continuity across potential spaces; its specific anatomical boundaries still require confirmation by targeted three-dimensional histological studies.

7.2. Composition and Contents of the Carotid Sheath

The carotid sheath is a tubular fascial compartment derived from the deep cervical fascia, extending from the aortic arch upward to the base of the skull. In the infrahyoid region, the sheath mainly contains the common carotid artery, internal jugular vein, and vagus nerve trunk. In the suprahyoid region, its contents expand to include the internal carotid artery, internal jugular vein, cranial nerves IX–XII, the ansa cervicalis, the sympathetic plexus, and deep cervical lymph nodes. The histological composition of the carotid sheath wall itself has been directly described. Hayashi’s analysis of semiserial sections of the carotid sheath from 8 sides of 5 cadavers showed that the sheath wall is composed of fibrous tissue, elastic tissue, and varying amounts of adipose tissue, and contains small nerves and microvessels; circular and oblique collagen fibers are present within the sheath wall, and a clear demarcation plane exists between the outer wall of the sheath and the surrounding cervical tissues [29]. Palliyalil et al. further confirmed in carotid sheath specimens from 30 patients with oral squamous cell carcinoma that the sheath wall is composed of fibroadipose tissue and scattered nerve bundles, and contains dilated lymphatic vessels and small lymph nodes [28]. Zhao et al.’s study supplemented the developmental histological evidence for a three-layered fascial architecture around the carotid sheath [27].

7.3. Coupling of Six Pathways

The common carotid artery and the internal carotid artery ascend within the sheath and supply blood to the brain. The carotid sinus is located within the sheath, and its baroreceptors monitor changes in blood pressure. Immunohistochemistry has confirmed that PIEZO1 and PIEZO2 are expressed in the sensory nerve structures of the human carotid sinus wall [4]. The circular and oblique arrangement of collagen fibers in the sheath wall [29] implies that the sheath wall has a directional mechanical response capacity. Shibasaki et al. used a muscle hardness meter and Doppler ultrasound to measure the relationship between trapezius muscle stiffness and the peak systolic velocity (PSV) of the transverse cervical artery, and found that PSV was significantly associated with neck and upper back stiffness, and that this association and low PSV were more pronounced in symptomatic subjects [47]. The study measured the transverse cervical artery rather than the carotid artery and assessed trapezius muscle hardness rather than purely fascial mechanical properties. Wang et al. demonstrated in a mouse model that the endothelial mechanosensitive cation channel PIEZO1 is required for flow-induced ATP release; ATP subsequently mediates phosphorylation and activation of AKT and eNOS through P2Y2/Gq/G11, ultimately regulating NO formation, vascular tone, and blood pressure [41]. The PIEZO1-specific activator Yoda1 can mimic the effects of fluid shear stress and induce vasodilation in a PIEZO1-dependent manner; endothelial-specific PIEZO1-deficient mice lose flow-induced NO formation and vasodilation and develop hypertension [41]. This means that if mechanical abnormalities in the fascia surrounding the carotid sheath can be transmitted to the vessel wall and activate endothelial PIEZO1, they could, in theory, simultaneously alter carotid blood flow through the “PIEZO1 → ATP → eNOS → NO → vasodilation” pathway. This evidence is derived from a mouse model, not from the human fascia surrounding the carotid sheath.

The internal jugular vein is the largest venous structure within the sheath and is located in its lateral portion. Elastic recoil of the sheath wall may prevent excessive collapse of the internal jugular vein during hypovolemia and help maintain cerebral venous return. Deep cervical lymph nodes are located within or immediately adjacent to the carotid sheath and serve as an important terminal for cervical lymphatic drainage. Pathological specimen studies have observed dilated lymphatic vessels and small lymph nodes in the carotid sheath [28]. More importantly, Clapham et al.’s bilateral dissection of the heads of four cadavers showed that of a total of 51 deep cervical lymph nodes, 12 were histologically confirmed to be located within the carotid sheath, directly adjacent to the internal carotid artery, with a mean diameter of 13.5 × 4.8 mm [30]. The study further noted that solutes and interstitial fluid in the brain can drain along the walls of cerebral arteries to these lymph nodes [30]. The glymphatic system refers to the cerebrospinal fluid–interstitial fluid exchange and metabolic waste clearance system driven along perivascular spaces. Fluid carrying cerebral metabolic waste drains through perivenous spaces to meningeal lymphatic vessels and ultimately empties into the deep cervical lymph nodes located within the carotid sheath. Clapham et al.’s findings provide direct anatomical evidence for this pathway [30].

The cervical sympathetic trunk is located posterior to the carotid sheath and is embedded in the prevertebral fascia. In the upper cervical region, the sympathetic trunk may enter the carotid sheath. In the human superior cervical ganglion, approximately 71% of neurons are PIEZO1-positive and 86% are PIEZO2-positive [4], suggesting that the sympathetic nervous system itself has mechanosensitivity. Systematic reviews have confirmed the presence of tyrosine hydroxylase-positive sympathetic nerve fibers within fascia [32], and Mense further provided quantitative data showing that approximately 40% of all innervation consists of these fibers [33]. The vagus nerve is the most consistently located neural structure within the carotid sheath, lying posterior to the common carotid artery and internal jugular vein. Fascial tension in the sheath wall may exert a functional influence on the vagus nerve. In the human petrosal ganglion, approximately 25% of neurons are PIEZO1-positive and 85% are PIEZO2-positive [4], suggesting that cranial sensory ganglia adjacent to the carotid sheath also possess mechanosensitive properties.

7.4. Strategic Position of the Fascial Integration Layer Surrounding the Carotid Sheath Within the Systemic Fascial Network

When the fascial integration layer surrounding the carotid sheath is examined within the systemic fascial network, its strategic position can be summarized as follows: it is the mechanical coat and functional anatomical integration layer of the vital structures of the neck, and the carotid sheath may be a candidate hub that integrates multiple key pathways within a single fascial conduit. This means that mechanical stress generated by abnormal head and neck posture is transmitted through the cervical fascial network to the carotid sheath and could, in theory, simultaneously affect arterial supply, venous return, lymphatic drainage, glymphatic clearance, sympathetic tone, and vagal function within the sheath. This mechanical transmission process has a quantifiable in vivo basis. Pires et al. confirmed that the mechanical pressure required to reach the human deep cervical fascia is approximately 6 N/cm² and that this pressure is not significantly affected by an individual’s mechanical pain threshold or subcutaneous fat thickness [9]. The finding that the cervical pressure pain threshold is significantly lower than that in the thoracolumbar region further supports the special position of the cervical fascia as a “highly sensitive mechanical node” within the systemic fascial network. The efficiency of external mechanical pressure transmission to the carotid sinus has been quantitatively assessed in human studies. Querry et al. confirmed in 19 subjects that external pressure can be effectively transmitted to carotid sinus tissue (balloon catheter method: negative pressure 82%, positive pressure 89%), and that the transmission characteristics remain stable at rest and during exercise [5]. This mechanical transmission pathway provides a biomechanical basis for the idea that “mechanical changes in the fascia surrounding the carotid sheath can affect carotid sinus function.” Combined with the expression of PIEZO1 and PIEZO2 proteins in the sensory nerve structures of the human carotid sinus wall [4], mechanical abnormalities in the fascia surrounding the carotid sheath could theoretically be transmitted to the carotid sinus wall through deformation of the sheath wall, thereby activating PIEZO channels and affecting baroreceptor function. However, it should be noted that Querry et al. measured tissue pressure inside the carotid sinus, not the mechanical response of fascial resident cells, and therefore cannot directly prove that fascial mechanical signals can activate PIEZO channels in fascial resident cells. The cellular and molecular mechanisms of this mechanical transmission pathway still require direct validation in human fascial resident cells surrounding the carotid sheath, namely P1.

Shibasaki et al.’s study further confirmed a significant association between neck and upper back stiffness and transverse cervical artery flow velocity, and this association and low PSV were more pronounced in symptomatic subjects [47], providing indirect clinical evidence from living humans that “mechanical abnormalities in the cervical fascia can affect cervical arterial blood flow.” Wang et al. completed the molecular mechanism of “PIEZO1 activation → altered blood flow” in a mouse model [41]. Nakao et al.’s study confirmed that after FHR, the shear modulus of the target fascia was significantly reduced in the stretched position but showed no change in the relaxed position [10], providing direct human biomechanical evidence that FHR is associated with a reduction in shear modulus, consistent with a change in the local mechanical environment. The stiffness of the cervical fascia itself has been reliably quantified by SWE, and posterior cervical fascial stiffness increases in the forward head posture [8], providing direct human evidence that “forward head posture is associated with altered mechanical properties of the cervical fascia.”

The premise for the fascial integration layer surrounding the carotid sheath to occupy a candidate strategic position within the systemic fascial network is that the systemic fascia indeed constitutes a functionally continuous mechanical network. Systematic review evidence of force transmission along myofascial chains [1] provides an empirical basis for this premise. This section will not repeat the evidence at each level; see the corresponding sections above for details. The holistic view requires viewing the neck as a strategic node within the systemic fascial network, rather than as an isolated anatomical region. More precisely, the fascial integration layer surrounding the carotid sheath is a candidate coupling interface of this strategic node.

8. Intervention: Fascial Hydrorelease as a Reset for Global Recovery

Fascial hydrorelease (FHR), also referred to as ultrasound-guided fascial hydrorelease (US-FHR) when performed under real-time ultrasound guidance, is a minimally invasive interventional procedure. Its core principle is to use fluid pressure to precisely separate adherent fascial layers, thereby restoring normal tissue gliding and relieving pain. During the procedure, under real-time monitoring with an ultrasound probe, the needle is inserted into the skin using an in-plane technique. When the needle tip reaches the target interfascial space, slow injection of the solution is initiated. A clear demarcation plane exists between the outer wall of the carotid sheath and the surrounding cervical tissues [29], providing a stable anatomical interface basis for effective fluid-pressure separation within the target interfascial space during FHR procedures. Diwan et al.’s cadaveric study further confirmed that injection of only 2 mL of fluid between the layers of the investing fascia can effectively spread and stain the nerve structures running between its layers [26], providing an operational reference for fluid-pressure separation during FHR. Zhao et al.’s developmental histology study further supports the existence of a multi-layered fascial architecture around the carotid sheath, which may provide a histological basis for interlayer fluid separation [27].

Based on the fascial memory hypothesis, the therapeutic mechanism of fascial hydrorelease can be understood as a “reset” of pathological fascial memory. Sustained mechanical stress leads to fascial fibrosis and adhesion, forming “stacked fascia” that appears as layered hyperechoic bands on ultrasound. This structural change may be accompanied by sustained activation of the YAP signaling pathway and related epigenetic modifications, constituting a candidate molecular basis for fascial memory [15]. The discovery of the Ang II–AT1R–YAP pathway [16] suggests that the RAS may participate in maintaining the pathological state of fascia; the finding that mechanical stretch activates the YAP pathway through PIEZO1 [17] further completes the molecular chain of “mechanotransduction → transcriptional program → fibrotic execution”; the discovery of the PIEZO1–ET-1–HIF-1α–TGF-β fibrotic pathway [18] reveals a positive feedback loop that sustains fascial fibrosis; evidence that hydrostatic pressure regulates fibroblast metabolism via the PIEZO1/YAP1/GLS1 axis [19] further suggests that the fluid pressure of FHR itself may regulate fibroblast activation status by affecting glutamine metabolism; and in vivo validation of the functional necessity of the PIEZO1–YAP/TAZ axis in fibrosis [20] further supports the rationale for PIEZO1–YAP/TAZ as candidate therapeutic targets. Wen et al. evaluated small-molecule modulators and nanotechnology-based intervention strategies targeting the Piezo1/YAP axis in a systematic review [21]. Liao et al. confirmed in human keloid fibroblasts that combined treatment with the Piezo1 inhibitor GsMTx4 and the YAP inhibitor verteporfin reduces keloid volume and decreases the ratio of type I to type III collagen [22], providing preclinical evidence from human cells for interventions targeting this axis. Shim et al. confirmed in human skin fibroblasts that mechanical stretch–induced changes in chromatin accessibility and H3K9me3 downregulation can be coordinated with changes in chromatin state through perinuclear actin assembly and emerin translocation [23]; this evidence is derived from skin fibroblasts, not fascial fibroblasts, suggesting that mechanical intervention may regulate epigenetic state by altering the nuclear mechanical environment. Tu et al. confirmed functional expression of PIEZO1/YAP1 at the fascial tissue level in a mouse model of arthritis [3], suggesting that fascial fibroblasts are important candidate cellular targets for mechanical intervention.

However, it must be clearly stated that the molecular mechanism of “memory reset” remains a hypothetical inference, and none of the existing RCTs have directly measured YAP/TAZ activity, epigenetic modifications, or molecular markers associated with fascial memory. The Chinese Expert Consensus on Ultrasound-guided Hydrodissection for Myofascial Trigger Points, released in 2026, also clearly states that this technique lacks large-scale, multicenter, double-blind randomized controlled trials and other highest-level evidence-based medical evidence, and that its analgesic mechanism has not yet been fully established [14]. Fascial hydrorelease physically restores tissue mobility and alters the local mechanical environment, potentially downregulating abnormal YAP activity and associated epigenetic changes and ultimately “resetting” this pathological fascial memory. Fascial resident cells possess mechanosensitive properties [2], suggesting that they may be important candidate cellular targets for this “reset” process. If the formation of fascial memory depends on specific parameter states of cell–ECM feedback dynamics [43], then the “reset” effect of fascial hydrorelease may depend not only on the degree of immediate mechanical release but also on whether postoperative local ECM viscoelasticity and cell mechanical feedback are pushed into a dynamic regime that does not maintain memory. In addition, if the mechanical state of fascia does indeed respond to changes in autonomic output on a timescale of several days [42], then the “reset” effect of fascial hydrorelease may also depend on the recovery trajectory of postoperative autonomic status.

FHR-induced changes in the mechanical properties of target fascia have been directly confirmed in human in vivo studies. Nakao et al. performed FHR in 12 patients with ultrasound-confirmed fascial thickening or stacking and used SWE to measure the shear modulus of muscle and fascia in the relaxed and stretched positions; they found that the shear modulus after FHR was significantly reduced in the stretched position but showed no change in the relaxed position [10]. This means that FHR does not simply “soften” tissue; rather, it reduces the tissue’s resistance to deformation when the tissue is under tensile load, that is, it reduces tissue stress. This provides direct quantitative human evidence that “FHR changes the local mechanical environment”; however, the causal relationship between FHR and “fascial memory reset” still requires direct molecular validation.

Fascial hydrorelease is currently used mainly to treat chronic pain caused by fascial adhesion and densification, including myofascial pain syndrome, restricted joint mobility, peripheral nerve entrapment, non-odontogenic tooth pain and orofacial pain, and thumb pain. FHR efficacy has been supported by evidence from randomized controlled trials. Suarez-Ramos et al. conducted a single-blind randomized controlled trial comparing ultrasound-guided interfascial hydrodissection (IH) with dry needling (DN), both combined with standardized shoulder exercise (SSE), in 46 patients with upper trapezius myofascial pain syndrome, assessing short- and long-term pain and quality-of-life outcomes. The results showed significant differences in visual analog scale (VAS) scores from baseline to each time point in both groups, but the effect size was greater in the IH + SSE group than in the DN + SSE group; on the EQ-5D-5L, the IH group showed greater effect sizes in the dimensions of usual activities, pain/discomfort, and anxiety/depression [11]. Overmann et al. conducted a meta-analysis including 10 RCTs and 549 patients with chronic neck pain, evaluating the effects of myofascial release on pain and range of motion. The results showed that myofascial release had a statistically significant effect on pain improvement (P = 0.03), with a moderate effect size, but no significant effect on pressure pain threshold improvement [45]. This study came from Schleip’s team, whose fascial mechanics framework is highly consistent with this hypothesis, providing systematic review–level evidence for the clinical efficacy of FHR. However, it should be noted that the meta-analysis included myofascial release interventions, not specifically FHR, and did not involve mechanotransduction molecules such as PIEZO1/YAP; therefore, it serves only as peripheral corroborating evidence for the clinical efficacy of FHR. Chen et al. conducted a randomized controlled trial including 41 patients with upper trapezius myofascial pain syndrome, comparing ultrasound-guided myofascial hydrodissection technique (UMHT) with lidocaine trigger point injection (TPI) in a direct comparison, with 12-week follow-up. The results showed that UMHT and TPI were both effective in reducing pain and improving function, with no statistically significant difference between the two groups (P > 0.05); the VAS effect size was r = 0.716 in the UMHT group and r = 0.604 in the TPI group [12]. Hiroki et al. conducted a prospective single-arm interventional study in patients with chronic cervical myofascial pain, performing ultrasound-guided fascial hydrorelease around the arteries supplying the neck, using 2 mL of saline for hydrodissection. The results showed that the VAS score during movement decreased significantly from a baseline of 49.0 mm; US-FHR was simple, safe, and associated with reduced neck pain [44]. This study was a single-arm, non-controlled design with a limited sample size, and the authors characterized the conclusions as hypothesis-generating. Farag et al. conducted a randomized controlled study including 120 patients with primary shoulder adhesive capsulitis (AC), comparing ultrasound-guided FHR with anterior hydrodilatation in a direct comparison, with follow-up to 12 months. The results showed that the FHR group had greater improvement in internal rotation, external rotation, and extension range of motion than the control group, and showed longer-term improvement in function and overall range of motion. The study concluded that ultrasound-guided FHR can effectively address pain and functional limitations, with greater durability in AC [13]. This study included patients with shoulder adhesive capsulitis, not patients with cervicogenic symptoms. The Chinese Expert Consensus on Ultrasound-guided Hydrodissection for Myofascial Trigger Points, released in 2026, clearly states that this technique has limitations such as insufficient high-quality evidence and that large-sample, multicenter randomized controlled trials are needed to promote evidence-based standardization and precision individualization [14]. Treatment regimens are usually individualized, with common patterns of once weekly for three to four sessions, or intervals adjusted according to treatment response. The efficacy reported in existing studies varies by condition and assessment criteria, generally showing positive short-term analgesia and functional improvement, but long-term efficacy and mechanisms still require higher-quality research for confirmation [11,12,13,14,45].

9. Testable Predictions

To standardize numbering and compress the text, all testable predictions are renumbered as P1–P24, each with a brief description of validation method, sample, and primary endpoint.

P1 (first priority): Expression of PIEZO1, PIEZO2, and YAP in resident cells of the human fascia surrounding the carotid sheath. Validation method: double-labeling immunohistochemistry. Sample: human cadaveric fascia surrounding the carotid sheath or discarded specimens from carotid endarterectomy (n ≥ 10). Primary endpoint: co-staining positivity rates of PIEZO1/2 with fibroblast markers (e.g., vimentin, CD34) or neural markers (e.g., PGP9.5). The hypothesized co-staining rate is >20% in at least one fibroblast marker. Positive control: human carotid sinus wall, which has confirmed PIEZO1/2 expression [4]. Antibodies: anti-PIEZO1, anti-PIEZO2, anti-YAP, anti-vimentin, anti-CD34, anti-PGP9.5. Representative literature is in Section 6.2. Among these, [3] is from a mouse model of arthritis, and [23] is from skin fibroblasts; neither is human fascia surrounding the carotid sheath. If this prediction is not confirmed, the hypothesis predicts that FHR effects will not correlate with fascial PIEZO1/2 expression and that YAP-dependent fibrotic markers will be absent in fascial resident cells.

P2: Fascia-specific adaptation in individuals with procedural motor skills. Validation method: SWE. Sample: long-term cyclists, swimmers, and touch typists. Primary endpoint: association between shear modulus in relevant fascial regions and feedback circuit dynamic parameters, possibly showing a non-monotonic relationship [43].

P3: Fascial responses elicited by conditioned stimuli. Validation method: pairing a neutral stimulus with cervical mechanical or painful stimulation. Sample: healthy subjects. Primary endpoint: changes in local fascial stiffness, local blood flow, pain scores, and heart rate variability.

P4: Association between the fascia surrounding the carotid sheath and multiple systems. Validation method: ultrasound elastography combined with Doppler flow measurement. Sample: patients with cervicogenic symptoms. Primary endpoint: correlations between fascial elasticity and carotid sheath blood flow parameters, vagal function indicators, and glymphatic drainage efficiency.

P5: Multisystem effects of fascial hydrorelease. Validation method: preoperative and postoperative comparison. Sample: patients with cervicogenic symptoms. Primary endpoint: synchronous reductions in conditioned pain responses, local CGRP concentration, and sympathetic tone, and their correlation with clinical symptom improvement.

P6: Temporal structure of autonomic–fascial coupling. Validation method: time-series design. Sample: patients with cervicogenic symptoms. Primary endpoint: delayed causal relationships in the fascia surrounding the carotid sheath similar to those reported by Brandl et al. [42].

P7: Cell–ECM feedback dynamics and maintenance of fascial memory. Validation method: primary fibroblast in vitro experiments. Sample: primary fibroblasts derived from the fascia surrounding the carotid sheath. Primary endpoint: regulation of mechanical stretch memory by cell–ECM recursive feedback, with non-monotonic dynamic characteristics [43].

P8: Myofascial chain force transmission and cervical mechanical coupling. Validation method: controlled mechanical loading applied to the plantar fascia or thoracolumbar fascia. Sample: healthy subjects. Primary endpoint: measurable mechanical responses in the region of the fascia surrounding the carotid sheath [1].

P9: Three-dimensional histological validation of fascial anatomical continuity. Validation method: E12 plastinated sections and confocal microscopy three-dimensional reconstruction. Sample: human cadaveric specimens. Primary endpoint: actual distribution pattern of the fascia surrounding the carotid sheath and its relationship to fascial discontinuities in the anterior cervical triangle [24] and posterior muscle spaces [25], as well as the continuity of the investing fascia bilaminar structure [26] and the three-layered fascial architecture around the carotid sheath [27].

P10: Molecular validation of YAP mechanical activation and fascial memory. Validation method: in vitro mechanical stretch, Ang II stimulation, PIEZO1 inhibitor, and GLS1 inhibitor interventions in primary fibroblasts. Sample: primary fibroblasts derived from the human fascia surrounding the carotid sheath. Primary endpoint: YAP nuclear translocation and transcriptional activity, ET-1/HIF-1α pathway activation, blockade of fibrotic progression in conditional gene knockout models, and mechanical stretch–induced changes in chromatin accessibility and H3K9me3 downregulation. Representative literature is in Section 6. Among these, [3] is from a mouse model of arthritis, and [23] is from skin fibroblasts.

P11: Association between mechanical parameters of the deep cervical fascia and multisystem function. Validation method: pressure algometer, ultrasound, and SWE. Sample: patients with cervicogenic symptoms. Primary endpoint: correlations between pressure required to reach the deep fascia, pressure pain threshold, cervical fascial stiffness, degree of forward head posture, and carotid sinus baroreceptor function. The hypothesized correlation coefficient is r > 0.4 between fascial stiffness and carotid flow parameters. Representative literature is in Section 7.

P12: Validation of fascial innervation and fascial reflexes. Validation method: immunohistochemical analysis. Sample: human fascia samples surrounding the carotid sheath. Primary endpoint: types and density of nerve endings, proportion of sympathetic nerve fibers, Ruffini corpuscles and Vater–Pacini corpuscles, laminar distribution of nociceptors, Pacinian-like and Ruffini-like mechanoreceptors, and PIEZO2 functional expression. Representative literature is in Section 5.

P13: Association between carotid sheath wall histology and mechanical function. Validation method: histological staining and mechanical testing. Sample: human carotid sheath specimens. Primary endpoint: correlation between circular and oblique collagen fiber orientation and mechanical anisotropy, and correlations of fibroadipose tissue, nerve bundles, dilated lymphatic vessels, and small lymph nodes with cervical lymphatic drainage and immune surveillance function [28,29].

P14: Lymphatic tissue within the carotid sheath and clearance of cerebral metabolic waste. Validation method: histological and tracer studies. Sample: human carotid sheath specimens. Primary endpoint: correlation between the presence and distribution of lymph nodes within the sheath and cerebral metabolic waste clearance pathways [30].

P15: Mechanical status of cervical soft tissue and hemodynamics. Validation method: ultrasound elastography and Doppler flow measurement. Sample: patients with cervicogenic symptoms. Primary endpoint: correlation between abnormal cervical fascial elasticity and blood flow parameters within the carotid sheath, and synchrony between improvement in fascial elasticity and improvement in carotid blood flow parameters after fascial hydrorelease [47].

P16: Quantitative validation of target fascial mechanical properties after FHR. Validation method: SWE. Sample: fascia surrounding the carotid sheath. Primary endpoint: significant reduction in shear modulus in the stretched position after FHR, with no significant change in the relaxed position. The hypothesized reduction in shear modulus is ≥15% in the stretched position after FHR [10].

P17: Hydrostatic pressure effects of FHR and the PIEZO1–YAP1–GLS1 pathway. Validation method: in vitro hydrostatic pressure stimulation of primary fibroblasts, with reference to the rat bladder fibroblast protocol described by Li et al. [19]. Sample: primary fibroblasts derived from the human fascia surrounding the carotid sheath. Primary endpoint: PIEZO1-dependent YAP1 nuclear translocation and GLS1 expression upregulation, and blocking effects of PIEZO1 inhibitors or GLS1 inhibitors.

P18: Randomized controlled validation of FHR clinical efficacy. Validation method: randomized controlled trial. Sample: patients with cervicogenic symptoms. Primary endpoint: greater effect size in pain scores and quality-of-life indicators in the FHR group, and synchrony with improvement in fascial elasticity parameters and carotid blood flow parameters, following the quality requirements of the 2026 Chinese expert consensus [11,12,14,45].

P19: In vivo functional validation of the PIEZO1–YAP/TAZ axis in fascial fibrosis. Validation method: conditional gene knockout. Sample: animal models of fascial fibrosis. Primary endpoint: significant inhibition of fibrotic progression after specific knockout of Piezo1 or Yap/Taz in fascial fibroblasts [20].

P20: Clinical efficacy validation of FHR in the cervical region. Validation method: randomized controlled trial. Sample: patients with cervicogenic symptoms. Primary endpoint: significant improvement in neck pain scores and quality-of-life indicators in the US-FHR group [44].

P21: 12-month RCT validation of long-term FHR efficacy. Validation method: randomized controlled trial with 12-month follow-up. Sample: patients with fascia-related diseases. Primary endpoint: longer-term improvement in function and overall range of motion in the FHR group [13].

P22: Equivalence validation of FHR and standard injection therapy. Validation method: randomized controlled trial. Sample: patients with cervicogenic symptoms. Primary endpoint: no statistically significant difference between FHR and lidocaine trigger point injection in pain reduction and functional improvement [12].

P23: Validation of investing fascia bilaminar structure and interlayer fluid separation. Validation method: ultrasound-guided dye injection and serial sectioning. Sample: human cadaveric specimens. Primary endpoint: dense bilaminar structure of the investing fascia and effective diffusion of small-volume fluid injected between layers, and corresponding structures in human specimens of the three-layered fascial architecture around the carotid sheath reported by Zhao et al. [26,27].

P24: Functional necessity of PIEZO1/YAP1 in fascial fibroblasts. Validation method: selective ablation or conditional gene knockout. Sample: fibroblasts derived from fascial tissue. Primary endpoint: significant attenuation of mechanical stimulation–induced ECM remodeling and pain relief effects, and downregulation of Piezo1 and Yap1 expression [3].

10. Boundaries and Limitations

10.1. Fascia Is Not the Hard Drive of Skill Memory

The memory subject for riding a bike, swimming, or touch typing remains the central nervous system. What the fascial hypothesis truly seeks to explain is why, once a motor program is formed, the body’s peripheral mechanical state tends to maintain this program and retains partial traces even after training cessation. This hypothesis distinguishes central procedural memory, conditioned reflex memory, peripheral fascial mechanical memory, and neuroplasticity-related memory, but these four types of memory are not equivalent in mechanism, and future research needs to further clarify their interconversion relationships.

10.2. The Subject of Conditioned Reflexes and Procedural Memory Is Central

Without the central nervous system, fascia cannot independently complete classical or operant conditioning. Fascia is the peripheral effector and mechanical memory substrate of conditioned reflexes.

10.3. Anatomical Controversy of the Investing Fascia

Nash et al. confirmed that the investing fascia does not exist in the anterior cervical triangle [24], and Zhang and Lee confirmed that the investing fascia does not exist between the sternocleidomastoid and trapezius muscles [25]. The discussion of fascial tissue surrounding the carotid sheath in this hypothesis should be understood as a functional description of fascial sources around the carotid sheath; Diwan et al.’s study further confirmed the bilaminar structure of the investing fascia and the feasibility of interlayer fluid separation [26], but that study used cadaveric specimens and cannot be fully equated with the living state. Zhao et al.’s developmental histology study confirmed in a rabbit model that a three-layered fascial architecture exists around the carotid sheath [27], but that study was not based on human specimens, and extrapolation should be approached with caution. It should be noted that the investing fascia is present on the surface of the trapezius and forms its muscle sheath; the discontinuity described by Zhang and Lee specifically refers to the fascial space between the sternocleidomastoid and trapezius muscles. Therefore, the expression “fascial integration layer surrounding the carotid sheath with investing fascia involvement” refers to the convergence and coupling of fascial sheets from different regions, including the trapezius fascia, around the carotid sheath, rather than a single, anatomically completely continuous fascial layer. The fascial composition of the carotid sheath, its attachment to the skull base, and its extension into the mediastinum are inconsistently described in the literature, and its anatomical boundaries still require targeted research for confirmation.

10.4. Evidence Levels, Source Limitations, and Core Open Questions

Evidence levels. The first level is direct human anatomical or clinical evidence, including evidence from the neck or carotid sheath region, with representative literature [1,4,5,6,7,8,9,10,11,12,13,24,25,26,28,29,30,31,32,44,47]. The second level is human evidence from non-neck regions, including tissue, cell, physiological, and systematic review evidence, with representative literature [15,16,22,23,34,35] (rat quantitative; human preliminary non-quantitative), [36,37,38,42,46]. The third level is animal model evidence, with representative literature [2,3,18,20,27,39,40,41]. The fourth level is in vitro evidence, with representative literature [43], rat bladder fibroblast evidence [19], and the L929 mouse fibroblast cell line [17]. The fifth level is expert consensus [14]. The interpretation boundaries of each level have been annotated in the corresponding sections above.

Core open questions. First, the P1 gap: expression of PIEZO1, PIEZO2, and YAP in resident cells of the human fascia surrounding the carotid sheath. The closest evidence comes from human carotid sinus wall sensory nerve structures [4], carotid atherosclerotic plaque myofibroblasts [6], and human thoracolumbar fascia [15], but none of these examined resident cells of the fascia surrounding the carotid sheath. Second, the fascial memory molecular gap: direct evidence of mechanical epigenetic modifications in human fascial fibroblasts. Shim et al. provided direct evidence in human skin fibroblasts [23], but not in fascial fibroblasts. Third, the FHR mechanism gap: direct in vitro validation of the effects of FHR fluid pressure on the PIEZO1/YAP pathway in fascial fibroblasts. Among existing evidence, the hydrostatic pressure–PIEZO1–YAP1–GLS1 pathway [19] is from cultured rat bladder fibroblasts, and the mechanical stretch–PIEZO1–YAP pathway [17] is from the L929 mouse fibroblast cell line; neither is under FHR-equivalent conditions in human fascial fibroblasts. These three gaps require original experimental research to fill, rather than supplementation at the level of literature retrieval. Future research should prioritize the immunohistochemical validation required by P1, while also conducting in vitro mechanical stretch and FHR-equivalent fluid pressure stimulation experiments in human fascial fibroblasts to detect epigenetic markers and PIEZO1/YAP pathway activation.

10.5. Causal Chains Are Largely Based on Inference

The causal chain constructed by this hypothesis—from fascial fibrosis to channel activation and then to downstream pathology—remains, to a large extent, a hypothetical framework based on literature inference rather than established fact.

Additional counterevidence and controversies. Several lines of evidence argue for caution. First, quantitative evidence for force transmission along myofascial chains in humans remains limited, and some lines (e.g., Superficial Front Line) have not shown demonstrable transition zones [1]. Second, the analgesic mechanism of FHR is still incompletely established, and the 2026 Chinese expert consensus notes a lack of highest-level evidence [14]. Third, the anatomical existence of the investing fascia is controversial; it is absent in the anterior cervical triangle and between the sternocleidomastoid and trapezius muscles [24,25], although it is present on the trapezius surface [26]. Fourth, the three-layered fascial architecture around the carotid sheath has been demonstrated only in a rabbit model, not in humans [27]. Fifth, direct evidence for mechanical epigenetic modifications in human fascial fibroblasts is still lacking; current evidence comes from skin fibroblasts [23]. These counterpoints do not invalidate the hypothesis but define its current evidence boundaries.

Alternative explanations. Alternative explanations, such as inflammatory signaling, vascular changes, or neural sensitization, may also contribute to the observed clinical effects and cannot be excluded by current evidence. The hypothesis does not claim exclusivity; rather, it proposes a testable pathway that may operate alongside these other mechanisms.

10.6. Clear Applicability Boundaries

This hypothesis applies only to the subgroup of primary headache patients with forward head posture, neck symptoms, and deep cervical fascial mechanical abnormalities, as well as to other cervicogenic symptoms with similar mechanical abnormalities. For patients without cervicogenic mechanical abnormalities, this hypothesis does not apply.

11. Conclusion

This hypothesis proposes that central encoding is the primary component, peripheral fascia is the mechanical memory substrate and reflex modulator, and the fascial integration layer surrounding the carotid sheath may be a candidate strategic hub coupling the systemic fascial network with vital organs. Its first-priority validation step is to directly confirm the expression of PIEZO1, PIEZO2, and YAP in resident cells of the human fascia surrounding the carotid sheath. The fascial memory molecular gap and the FHR mechanism gap are juxtaposed with it, constituting a first-priority experimental roadmap that requires original experimental research to fill. Before key evidence is obtained, the entire hypothesis should be regarded as a research program with clear falsifiability, rather than an established pathophysiological mechanism. If P1 is not confirmed, the fascial resident cell component would lack direct molecular support, but the broader pathway could still be partially supported by PIEZO1/2 expression from other cellular sources.